In Japan, most paging systems transmit pages more than once to improve paging performance. In general, the multiple transmissions of a message are spaced apart in time. Since a pager on a repeat system is given multiple opportunities to receive the same message in potentially different RF environments (due to the variance of field strength over time and the time difference of transmissions), the pager will gain paging sensitivity. This concept is generally known as time-diversity.
Depending on a selective call receiver's (or pager's) knowledge of the timing of a message's repeats, the pager can "build" a message. Messages can be "built" at various levels within a messaging protocol structure or scheme, including at the page level, at the codeword level, or at the bit level. If a pager knows the timing (128 frames per cycle in Motorola's FLEX.TM. high speed paging protocol) in which it expects the repeats of a page, it can "build" the message at the page level by selecting the best message of all the repeats it receives. Sequential lockout is used to eliminate multiple alerts for the same message when a preceding message in a time diversity system is received error free. For instance, a pager in a POCSAG system where the message is repeated three times would ignore the second and third repeats if the message was decoded error free on the first repeat or ignore just the third repeat if the message was decoded on the second repeat.
If a pager knows the time frame in which it expects the repeats of a page and can be guaranteed that pages received during that time frame are repeats, it can "build" the message at the codeword level by combining the best message codewords of all the repeats it receives. This type of "building" is referred to as message codeword combining (MCWC) and is currently used in the NTT 1200 signaling scheme in Japan. Finally, if a pager knows the exact time it expects the repeat of a page, it can "build" the page at the codeword level by combining the best of all codewords of all the repeats . Alternatively, the pager could "build" the page at the bit level by combining the best bits of all the codewords (including address and/or vectors) of all the repeats. These two types of "building" are currently supported by FLEX.TM.-TD, the version of FLEX.TM. used in Japan. They are referred to as codeword combining (CWC) and bit combining (BC).
The disadvantage of such a system is that all messages in a frame are repeated the same number of times, whether they need to be or not. Further, addresses, vectors, and message fragments are all repeated the same number of times, even if they have different length or importance. Hence, what is needed is a different manner of incorporating repeated messages that is preferably backwards compatible with Motorola's Flex.TM. protocol that can avoid introducing new flags or vectors in existing protocols and allows mixing with non-repeat pagers. Further, the new protocol should allow for different number of repeats for different messages and preferably allows different repeat factors for addresses and data for the same message fragment.